The Molecular Cloud Life Cycle. I. Constraining H₂ Formation and Dissociation Rates with Observations

Abstract

Molecular clouds (MCs) are the birthplaces of new stars in galaxies. A key component of MCs are photodissociation regions (PDRs), where far-ultraviolet radiation plays a crucial role in determining the gas’s physical and chemical state. Traditional PDR models assume a chemical steady state (CSS), where the rates of H2 formation and photodissociation are balanced. However, real MCs are dynamic and can be out of CSS. In this study, we demonstrate that combining H2 emission lines observed in the far-ultraviolet or infrared with column density observations can be used to derive the rates of H2 formation and photodissociation. We derive analytical formulae that relate these rates to observable quantities, which we validate using synthetic H2 line emission maps derived from the SILCC-Zoom hydrodynamical simulation. Our method estimates integrated H2 formation and dissociation rates with an accuracy ≈30% (on top of the uncertainties in the observed H2 emission maps and column densities). Our simulations, valid for column densities N ≤ 2 × 1022 cm−2, cover a wide dynamic range of H2 formation and photodissociation rates, showing significant deviations from CSS, with 74% of the MC’s mass deviating from CSS by a factor greater than 2. Our analytical formulae can effectively distinguish between regions in and out of CSS. When applied to actual H2 line observations, our method can assess the chemical states of MCs, providing insights into their evolutionary stages and lifetimes. A NASA Small Explorer mission concept, Eos, will be proposed in 2025 and is specifically designed to conduct the types of observations outlined in this study.